1. Field of the Invention
The present invention relates to a method for dynamic and non-contact measurement of a displacement of a grounded conductive substance with respect to a capacitive sensor and to a method for dynamic and non-contact measurement of the permittivity of a dielectric substance between a grounded conductive part and a capacitive sensor. The invention relates more particularly to a method for dynamic and non-contact measurement of a displacement which can be used favourably to permanently measure with relative simplicity the axial displacement of a shaft of a rotating machine or to measure a fluid level in a tank, and also to a method for dynamic and non-contact measurement of the permittivity of a dielectric substance which can be used favourably to also measure a fluid level in a tank or continuously monitor with relative simplicity a possible change in the composition of a substance flowing through a conduit.
2. Description of the Prior Art
U.S. Pat. No. 4,675,670 granted to HYDRO-QUEBEC describes an apparatus and a method for dynamic and non-contact measurement of the distance separating the surface of a first part, that may be conductive or not, from the surface of a second conductive part closely spaced from the first part and grounded, such as the stator and rotor of an electric generator. The apparatus and method can be permanently used without significant modification or excessive congestion, while providing precise and reliable results even in the presence of intense magnetic fields or temperature variations.
The above-mentioned apparatus includes a sensor made of two parallel conductive plates, superimposed and electrically insulated from one another, and fed by a high frequency signal between 100 kHz and 10 MHz at a predetermined voltage between 5 and 100 volts, connected to a device for detecting a current value, which is itself connected to a device which processes the detected current value, such as a computer.
The sensor during its use forms a capacitor with the grounded conductive part, so that the capacitance is determined by the following known equation: ##EQU1## in which: K=.epsilon.o.epsilon.r, .epsilon.o being the vacuum permittivity (8.854 pF/m) and .epsilon.r being the relative permittivity of the dielectric substance between the nearest sensor plate from the conductive part and this conductive part;
Ar is the overlapping surface of the conductive part on the sensor plate; and PA1 D is the distance between the surface of the nearest sensor plate from the conductive part and this conductive part. PA1 .omega.=2.pi.f, f being the frequency of the emitted signal; PA1 V is the voltage difference between the nearest sensor plate from the conductive part and this conductive part; and PA1 C is the above-mentioned capacitance. PA1 (a) positioning the capacitive sensor at a fixed distance close to the conductive substance, the plates of the capacitive sensor being parallel to the plane in which the conductive substance extends, such that a displacement of this substance in the mentioned plane modifies an overlapping surface formed by portions of the conductive substance and the capacitive sensor which are superimposed; PA1 (b) detecting the current induced by a high frequency signal in the capacitive sensor, this current having a value varying in a directly proportional relationship with the overlapping surface; and PA1 (c) determining the value of the displacement of the conductive substance in respect with the capacitive sensor according to the value of the current. PA1 (a) positioning the capacitive sensor at a fixed distance close to the conductive part so that the dielectric substance whose permittivity is to be measured is between the conductive part and the capacitive sensor; PA1 (b) detecting the current induced by a high frequency signal into the capacitive sensor, this detected current varying in a directly proportional relationship with the permittivity of the dielectric substance; and PA1 (c) determining the value of the permittivity of the dielectric substance between the conductive part and the capacitive sensor according to the value of the current.
When the so formed capacitor is subjected to a high frequency signal, a measureable current is induced in the sensor plates, of which the intensity responds in accordance with the following equation: EQU i=.omega.Cv (2)
in which:
Equation (1) shows that for constant dielectric value K and overlapping surface Ar, the capacitance C, and so the current i of equation (2), varies according to the inverse of the distance D separating the sensor from the conductive part, making possible the mentioned method for dynamic and non-contact measurement of the distance between the nearest capacitive sensor plate from a conductive part and the conductive part.
As it can be easily seen, the apparatus can be similarly used to carry out the measurement of another variable parameter in equation (1), such as the permittivity K or the overlapping surface Ar for instance, as long as the other parameters are fixed.